[LOOPINFO] Extend Loop object to add utilities to get the loop bounds,

step, and loop induction variable.

Summary: This PR extends the loop object with more utilities to get loop
bounds, step, and loop induction variable. There already exists passes
which try to obtain the loop induction variable in their own pass, e.g.
loop interchange. It would be useful to have a common area to get these
information.

/// Example:
/// for (int i = lb; i < ub; i+=step)
///   <loop body>
/// --- pseudo LLVMIR ---
/// beforeloop:
///   guardcmp = (lb < ub)
///   if (guardcmp) goto preheader; else goto afterloop
/// preheader:
/// loop:
///   i1 = phi[{lb, preheader}, {i2, latch}]
///   <loop body>
///   i2 = i1 + step
/// latch:
///   cmp = (i2 < ub)
///   if (cmp) goto loop
/// exit:
/// afterloop:
///
/// getBounds
///   getInitialIVValue      --> lb
///   getStepInst            --> i2 = i1 + step
///   getStepValue           --> step
///   getFinalIVValue        --> ub
///   getCanonicalPredicate  --> '<'
///   getDirection           --> Increasing
/// getInductionVariable          --> i1
/// getAuxiliaryInductionVariable --> {i1}
/// isCanonical                   --> false

Reviewers: kbarton, hfinkel, dmgreen, Meinersbur, jdoerfert, syzaara,
fhahn
Reviewed By: kbarton
Subscribers: tvvikram, bmahjour, etiotto, fhahn, jsji, hiraditya,
llvm-commits
Tag: LLVM
Differential Revision: https://reviews.llvm.org/D60565

llvm-svn: 362644

1 files changed, 214 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/LoopInfo.cpp b/llvm/lib/Analysis/LoopInfo.cpp
index aa933d98f24..00dbe30c2b3 100644
--- a/llvm/lib/Analysis/LoopInfo.cpp
+++ b/llvm/lib/Analysis/LoopInfo.cpp
@@ -17,10 +17,12 @@
 #include "llvm/ADT/DepthFirstIterator.h"
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/Analysis/IVDescriptors.h"
 #include "llvm/Analysis/LoopInfoImpl.h"
 #include "llvm/Analysis/LoopIterator.h"
 #include "llvm/Analysis/MemorySSA.h"
 #include "llvm/Analysis/MemorySSAUpdater.h"
+#include "llvm/Analysis/ScalarEvolutionExpressions.h"
 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/IR/CFG.h"
@@ -164,6 +166,218 @@ PHINode *Loop::getCanonicalInductionVariable() const {
   return nullptr;
 }
 
+/// Get the latch condition instruction.
+static ICmpInst *getLatchCmpInst(const Loop &L) {
+  if (BasicBlock *Latch = L.getLoopLatch())
+    if (BranchInst *BI = dyn_cast_or_null<BranchInst>(Latch->getTerminator()))
+      if (BI->isConditional())
+        return dyn_cast<ICmpInst>(BI->getCondition());
+
+  return nullptr;
+}
+
+/// Return the final value of the loop induction variable if found.
+static Value *findFinalIVValue(const Loop &L, const PHINode &IndVar,
+                               const Instruction &StepInst) {
+  ICmpInst *LatchCmpInst = getLatchCmpInst(L);
+  if (!LatchCmpInst)
+    return nullptr;
+
+  Value *Op0 = LatchCmpInst->getOperand(0);
+  Value *Op1 = LatchCmpInst->getOperand(1);
+  if (Op0 == &IndVar || Op0 == &StepInst)
+    return Op1;
+
+  if (Op1 == &IndVar || Op1 == &StepInst)
+    return Op0;
+
+  return nullptr;
+}
+
+Optional<Loop::LoopBounds> Loop::LoopBounds::getBounds(const Loop &L,
+                                                       PHINode &IndVar,
+                                                       ScalarEvolution &SE) {
+  InductionDescriptor IndDesc;
+  if (!InductionDescriptor::isInductionPHI(&IndVar, &L, &SE, IndDesc))
+    return None;
+
+  Value *InitialIVValue = IndDesc.getStartValue();
+  Instruction *StepInst = IndDesc.getInductionBinOp();
+  if (!InitialIVValue || !StepInst)
+    return None;
+
+  const SCEV *Step = IndDesc.getStep();
+  Value *StepInstOp1 = StepInst->getOperand(1);
+  Value *StepInstOp0 = StepInst->getOperand(0);
+  Value *StepValue = nullptr;
+  if (SE.getSCEV(StepInstOp1) == Step)
+    StepValue = StepInstOp1;
+  else if (SE.getSCEV(StepInstOp0) == Step)
+    StepValue = StepInstOp0;
+
+  Value *FinalIVValue = findFinalIVValue(L, IndVar, *StepInst);
+  if (!FinalIVValue)
+    return None;
+
+  return LoopBounds(L, *InitialIVValue, *StepInst, StepValue, *FinalIVValue,
+                    SE);
+}
+
+using Direction = Loop::LoopBounds::Direction;
+
+ICmpInst::Predicate Loop::LoopBounds::getCanonicalPredicate() const {
+  BasicBlock *Latch = L.getLoopLatch();
+  assert(Latch && "Expecting valid latch");
+
+  BranchInst *BI = dyn_cast_or_null<BranchInst>(Latch->getTerminator());
+  assert(BI && BI->isConditional() && "Expecting conditional latch branch");
+
+  ICmpInst *LatchCmpInst = dyn_cast<ICmpInst>(BI->getCondition());
+  assert(LatchCmpInst &&
+         "Expecting the latch compare instruction to be a CmpInst");
+
+  // Need to inverse the predicate when first successor is not the loop
+  // header
+  ICmpInst::Predicate Pred = (BI->getSuccessor(0) == L.getHeader())
+                                 ? LatchCmpInst->getPredicate()
+                                 : LatchCmpInst->getInversePredicate();
+
+  if (LatchCmpInst->getOperand(0) == &getFinalIVValue())
+    Pred = ICmpInst::getSwappedPredicate(Pred);
+
+  // Need to flip strictness of the predicate when the latch compare instruction
+  // is not using StepInst
+  if (LatchCmpInst->getOperand(0) == &getStepInst() ||
+      LatchCmpInst->getOperand(1) == &getStepInst())
+    return Pred;
+
+  // Cannot flip strictness of NE and EQ
+  if (Pred != ICmpInst::ICMP_NE && Pred != ICmpInst::ICMP_EQ)
+    return ICmpInst::getFlippedStrictnessPredicate(Pred);
+
+  Direction D = getDirection();
+  if (D == Direction::Increasing)
+    return ICmpInst::ICMP_SLT;
+
+  if (D == Direction::Decreasing)
+    return ICmpInst::ICMP_SGT;
+
+  // If cannot determine the direction, then unable to find the canonical
+  // predicate
+  return ICmpInst::BAD_ICMP_PREDICATE;
+}
+
+Direction Loop::LoopBounds::getDirection() const {
+  if (const SCEVAddRecExpr *StepAddRecExpr =
+          dyn_cast<SCEVAddRecExpr>(SE.getSCEV(&getStepInst())))
+    if (const SCEV *StepRecur = StepAddRecExpr->getStepRecurrence(SE)) {
+      if (SE.isKnownPositive(StepRecur))
+        return Direction::Increasing;
+      if (SE.isKnownNegative(StepRecur))
+        return Direction::Decreasing;
+    }
+
+  return Direction::Unknown;
+}
+
+Optional<Loop::LoopBounds> Loop::getBounds(ScalarEvolution &SE) const {
+  if (PHINode *IndVar = getInductionVariable(SE))
+    return LoopBounds::getBounds(*this, *IndVar, SE);
+
+  return None;
+}
+
+PHINode *Loop::getInductionVariable(ScalarEvolution &SE) const {
+  if (!isLoopSimplifyForm())
+    return nullptr;
+
+  BasicBlock *Header = getHeader();
+  assert(Header && "Expected a valid loop header");
+  ICmpInst *CmpInst = getLatchCmpInst(*this);
+  if (!CmpInst)
+    return nullptr;
+
+  Instruction *LatchCmpOp0 = dyn_cast<Instruction>(CmpInst->getOperand(0));
+  Instruction *LatchCmpOp1 = dyn_cast<Instruction>(CmpInst->getOperand(1));
+
+  for (PHINode &IndVar : Header->phis()) {
+    InductionDescriptor IndDesc;
+    if (!InductionDescriptor::isInductionPHI(&IndVar, this, &SE, IndDesc))
+      continue;
+
+    Instruction *StepInst = IndDesc.getInductionBinOp();
+
+    // case 1:
+    // IndVar = phi[{InitialValue, preheader}, {StepInst, latch}]
+    // StepInst = IndVar + step
+    // cmp = StepInst < FinalValue
+    if (StepInst == LatchCmpOp0 || StepInst == LatchCmpOp1)
+      return &IndVar;
+
+    // case 2:
+    // IndVar = phi[{InitialValue, preheader}, {StepInst, latch}]
+    // StepInst = IndVar + step
+    // cmp = IndVar < FinalValue
+    if (&IndVar == LatchCmpOp0 || &IndVar == LatchCmpOp1)
+      return &IndVar;
+  }
+
+  return nullptr;
+}
+
+bool Loop::getInductionDescriptor(ScalarEvolution &SE,
+                                  InductionDescriptor &IndDesc) const {
+  if (PHINode *IndVar = getInductionVariable(SE))
+    return InductionDescriptor::isInductionPHI(IndVar, this, &SE, IndDesc);
+
+  return false;
+}
+
+bool Loop::isAuxiliaryInductionVariable(PHINode &AuxIndVar,
+                                        ScalarEvolution &SE) const {
+  // Located in the loop header
+  BasicBlock *Header = getHeader();
+  if (AuxIndVar.getParent() != Header)
+    return false;
+
+  // No uses outside of the loop
+  for (User *U : AuxIndVar.users())
+    if (const Instruction *I = dyn_cast<Instruction>(U))
+      if (!contains(I))
+        return false;
+
+  InductionDescriptor IndDesc;
+  if (!InductionDescriptor::isInductionPHI(&AuxIndVar, this, &SE, IndDesc))
+    return false;
+
+  // The step instruction opcode should be add or sub.
+  if (IndDesc.getInductionOpcode() != Instruction::Add &&
+      IndDesc.getInductionOpcode() != Instruction::Sub)
+    return false;
+
+  // Incremented by a loop invariant step for each loop iteration
+  return SE.isLoopInvariant(IndDesc.getStep(), this);
+}
+
+bool Loop::isCanonical(ScalarEvolution &SE) const {
+  InductionDescriptor IndDesc;
+  if (!getInductionDescriptor(SE, IndDesc))
+    return false;
+
+  ConstantInt *Init = dyn_cast_or_null<ConstantInt>(IndDesc.getStartValue());
+  if (!Init || !Init->isZero())
+    return false;
+
+  if (IndDesc.getInductionOpcode() != Instruction::Add)
+    return false;
+
+  ConstantInt *Step = IndDesc.getConstIntStepValue();
+  if (!Step || !Step->isOne())
+    return false;
+
+  return true;
+}
+
 // Check that 'BB' doesn't have any uses outside of the 'L'
 static bool isBlockInLCSSAForm(const Loop &L, const BasicBlock &BB,
                                DominatorTree &DT) {